The flavor of a green tea depends on the “varietal” of the tea plant, where it was harvested, how it was processed, and the harvest season. To maintain the green color of the leaves and the characteristic of the tea, the leaves have to go through a heating process after they're picked to stop the oxidation.
Traditional Japanese green teas are steamed, while traditional Chinese green teas are pan-fired (or heated in large rotating drums in the larger facilities). This gives Japanese greens a more grassy flavor and aroma, while Chinese greens are more vegetal.
Steep your green tea in cooler water (175 to 185 degrees) to avoid adding bitterness.